Explicitly annot virtual/conf packages on installation

[kit-ty-kate]

From a discussion on the OCaml Discord, someone was surprised to see janestreet base as a dependency of their package when using a lock file.

Upon looking at it @mseri realized the user was talking about base-bytes, which is not a janestreet base subpackage but simply a virtual package denoting the availibility of the bytes feature in the compiler.

I think it would be worth marking them explicitly as virtual when installing the package, the same way it is done when pinning (see #4987). Such thing could look like:

<><> Installing new switch packages <><><><><><><><><><><><><><><><><><><><><><>
Switch invariant: ["ocaml-system"]
The following actions will be faked:
  - install base-bigarray base (virtual)   [required by opam-core]
  - install base-threads  base (virtual)   [required by dune]
  - install base-unix     base (virtual)   [required by opam-core]
  - install ocaml-system  4.08.0
  - install conf-m4       1 (virtual)      [required by ocamlfind]
  - install ocaml-config  1                [required by ocaml]
  - install ocaml         4.08.0 (virtual) [required by opam-core]
  - install seq           base (virtual)   [required by re]
  - install ocamlfind     1.8.1            [required by ocamlgraph]
  - install dune          2.5.1            [required by opam-core]
  - install ocamlgraph    1.8.8            [required by opam-core]
  - install re            1.9.0            [required by opam-core]
  - install cppo          1.6.6            [required by opam-core]
  - install opam-core     2.0.7 (pinned)
===== 14 to install =====